Interesting Inverts – Emerald Crabs (Mithraculus sculptus)

The Emerald or Mithrax Crab is sometimes (but not frequently anymore) called the Green Clinging Crab, which just wasn't all that imaginative. They are native to the tropical Caribbean / Gulf of Mexico, ranging from the Bahamas and South Florida, down to the waters of Northern Brazil. Their glossy green coloration blends them into the bubble algae and coral environments they frequently live in. Their diminutive size (less than 1.5 inches usually) belies their huge appetites for just about everything. Long gone are the days where we see them as obligate bubble algae eaters as we've come to understand their role is more as an all-around detrivore, which still makes them a very attractive addition to the reef aquarium. When well fed, they are the model reef safe invertebrate.

In The Aquarium

One day you noticed a pretty green bubble growing on a piece of live rock, or a coral plug. If you were a seasoned aquarist, you knew enough to panic, drop everything, and remove that pestilence with haste. If you weren't, you soon saw an aquarium brimming with those little bubbles like a skimmer run amok. Regardless of which camp you fall into, your tank is a good candidate for an emerald crab or five. Emerald crabs are generally good reef inhabitants, they will consume bubble algae and it doesn't take too many of them to keep it at bay. Like most other natural controls, no number of them is likely to be able to bring a full scale outbreak back down to stability, but they are efficient at maintenance after manual removal of the nuisance algae

The trade off is that they will need supplemental foods (we like a meaty mix here) because if they are allowed to go hungry, they will hunt for coral polyps, clam mantles, sleeping fish, and just about anything else they can get their claws on. We don't recommend adding more than 1 for every 20 gallons or so as they can get feisty with each other, however, with ample space and food, having them spawn in the aquarium isn't unheard of. This is a great treat for all the corals and small fish in your tank. The only other consideration for keeping them that you'll need to weigh is that these are a preferred food of pufferfish, tuskfish and triggers. If adding any type of crab to an aquarium that houses these species, you should expect to regularly restock the crabs. Like all invertebrates they are intolerant of copper, so avoid any treatments that include it.

Biblio:

Crab Database, Resource Library. https://www.crabdatabase.info/en/crabs/brachyura/eubrachyura/heterotremata/majoidea/mithracidae/mithraculus/mithraculus-sculptus-4359. Accessed 10 September 2020.

WORMS, Davie, Peter (2013). http://www.marinespecies.org/aphia.php?p=taxdetails&id=421990. Accessed 10 September 2020